The cosmic timeline traces the universe's evolution from the Big Bang to today. It spans epochs like the Planck, Inflationary, and Quark eras, each marking crucial stages in the cosmos's development. These periods shaped the universe we see now.

Key events like inflation, nucleosynthesis, and galaxy formation punctuate this timeline. Evidence supporting this chronology includes the cosmic microwave background, light element abundances, and the expansion of the universe as seen through galactic redshifts.

The Cosmic Timeline

Epochs of universal history

• Planck Epoch
  • Earliest stage of the universe lasting from the moment of the Big Bang to $10^{-43}$ seconds
  • Physics at this scale not well understood due to extreme density and temperature conditions (Planck scale)
• Grand Unification Epoch
  • Occurred between $10^{-43}$ and $10^{-36}$ seconds after the Big Bang
  • Fundamental forces except gravity were unified into a single force
• Inflationary Epoch
  • Rapid exponential expansion of the universe lasting from $10^{-36}$ to $10^{-32}$ seconds after the Big Bang
  • Responsible for large-scale homogeneity and isotropy of the universe (smoothness and uniformity)
• Electroweak Epoch
  • Occurred between $10^{-32}$ and $10^{-12}$ seconds after the Big Bang
  • Electroweak force separated into electromagnetic and weak nuclear forces
• Quark Epoch
  • Lasted from $10^{-12}$ to $10^{-6}$ seconds after the Big Bang
  • Quarks and gluons formed a hot, dense quark-gluon plasma
• Hadron Epoch
  • Occurred between $10^{-6}$ and 1 second after the Big Bang
  • Quarks combined to form hadrons such as protons and neutrons
• Lepton Epoch
  • Lasted from 1 second to 10 seconds after the Big Bang
  • Leptons dominated the universe (electrons, positrons, neutrinos)
• Photon Epoch
  • Occurred between 10 seconds and 380,000 years after the Big Bang
  • Universe was a hot, dense plasma of nuclei, electrons, and photons
• Matter-Radiation Equality
  • Approximately 50,000 years after the Big Bang
  • Energy density of matter and radiation were equal
• Recombination and Decoupling
  • Occurred around 380,000 years after the Big Bang
  • Electrons combined with nuclei to form neutral atoms and photons decoupled from matter (cosmic microwave background)
• Dark Ages
  • Lasted from 380,000 years to about 400 million years after the Big Bang
  • No stars or galaxies had formed yet, universe was dark
• Reionization
  • Began around 400 million years after the Big Bang
  • First stars and galaxies formed, reionizing the neutral hydrogen in the universe
• Galaxy Formation and Evolution
  • Ongoing since the end of the Dark Ages
  • Galaxies continue to form, evolve, and interact through mergers and accretion
• Present Day
  • Approximately 13.8 billion years after the Big Bang
  • Universe continues to expand and evolve (dark energy dominates)

Key events in cosmic evolution

• Inflation
  • Rapid exponential expansion of the universe during the Inflationary Epoch
  • Solves horizon, flatness, and monopole problems in cosmology
  • Quantum fluctuations during inflation seeded formation of large-scale structures (galaxies, clusters)
• Nucleosynthesis
  • Occurred during the Lepton Epoch, between 1 second and 10 seconds after the Big Bang
  • Light elements formed (deuterium, helium-3, helium-4, lithium-7)
  • Relative abundances of these elements provide evidence for the Big Bang theory
• Galaxy Formation
  • Began during the Reionization epoch, around 400 million years after the Big Bang
  • Gravitational collapse of matter overdensities formed the first stars and galaxies
  • Galaxies continue to form, evolve, and interact through processes such as:
    1. Mergers
    2. Accretion of gas and smaller galaxies

Redshift and universal expansion

• Redshift
  • Increase in wavelength of electromagnetic radiation as it travels through space
  • Caused by expansion of the universe stretching photon wavelengths
  • Measured as $z = (\lambda_{observed} - \lambda_{emitted}) / \lambda_{emitted}$
• Relationship to the expansion of the universe
  • Redshift is a direct consequence of the expansion of the universe
  • Farther galaxies have greater redshift (Hubble's Law)
  • Hubble's Law: $v = H_0 \times d$, where $v$ is recessional velocity, $H_0$ is Hubble constant, and $d$ is distance to the galaxy
  • Redshift measurements provide evidence for expansion and can estimate distances to galaxies

Evidence for cosmic timeline

• Cosmic Microwave Background (CMB) Radiation
  • Relic radiation from the early universe, originating during Recombination and Decoupling epoch
  • Nearly uniform background radiation with blackbody spectrum at 2.7 K
  • Provides snapshot of universe at 380,000 years after the Big Bang
  • Tiny fluctuations in CMB temperature correspond to density fluctuations that seeded large-scale structure formation
• Big Bang Nucleosynthesis (BBN)
  • Predicts relative abundances of light elements formed in the early universe (deuterium, helium-3, helium-4, lithium-7)
  • Observational measurements of these abundances agree with BBN predictions, supporting the Big Bang theory
• Hubble's Law and the Expansion of the Universe
  • Galaxies observed to be receding from Earth, with more distant galaxies receding faster
  • Consistent with an expanding universe that originated from a single point (Big Bang)
  • Measured using redshift surveys of galaxies
• Age of the Oldest Stars and Galaxies
  • Oldest observed stars and galaxies consistent with age of universe predicted by cosmic timeline
  • Globular clusters contain some of the oldest stars, with ages around 12-13 billion years
  • Most distant galaxies observed have ages consistent with forming during Reionization epoch